The libraries primarily contribute to the object tool. But libraries contain a lot more than just objects. There are images that are applied to attributes and other classifications of GDL parts not accessible by the object tools settings.
Some of these “Other GDL” parts are components, or objects that can be applied to another object. Most of us are familiar with creating custom door leafs and window sashes, these are components of the door and window tool. But there are other classifications too, elements saved to the library that can be applied to many other tools.
One example that many are not aware of or do not think about, is the lamp tool. Lamps are just GDL objects that have been saved with a subtype that restricts access to the Lamp tool settings, rather than the Object tool. Some of these lamps are basic light sources, great for renderings. Others are actual fixtures, usually very generic in shape and appearance. These can be a great resource into creating a convincing image or rendering without spending the time to model or find a generic shaped lamp. As an added bonus, these objects almost always have a light source, intensity and color associated with them. This can greatly improve the results of your rendering when the lamp settings are turned on in the rendering settings palette.
Some of the lamps that work very well for generic visualization purposes are the ceiling fixture, pendant fixture and sconce lamps. Other lamps in the default libraries do not have many shape options other than size and surface.